This invention relates to the recovery of HF in increased yield from spent aluminum reduction cell linings. More particularly, it concerns an improved pyrohydrolysis method for the recovery of valuable components from spent aluminum reduction cell linings.
It is known that spent aluminum reduction cell linings contain a significant quantity of carbonaceous material, a mixture of fluoridic salts and Al.sub.2 O.sub.3. Several methods have already been recommended for the recovery of the fluoridic salt and alumina content of this spent material. One of the most efficient methods involves the pyrohydrolysis of the spent cell or pot lining in a fluidized bed reactor. Pyrohydrolysis involves contacting a fluidized bed of spent lining with water or steam and the H.sub.2 O introduced reacts with the fluoridic compounds to form HF.
The presence of sodium fluoride and other sodium containing compounds in the pot lining results in the formation of sodium fluoride and sodium oxide vapors. The sodium fluoride vapors are generated from the feed material by the high temperature required in the reactor. The sodium oxide vapors are the initial product of the pyrohydrolysis reactions and the decomposition product of sodium carbonate present in the feed. The sodium oxide vapors react with alumina present in the charge to form an Na.sub.2 O.xAl.sub.2 O.sub.3 compound which remains in the clinker discharged from the bed after completion of the pyrohydrolysis reaction. The constant generation of sodium fluoride and sodium oxide vapors within the fluidized bed and the short gas residence time in the bed results in a reactor offgas which contains a significant quantity of sodium fluoride and sodium oxide vapors. Upon cooling the sodium oxide vapors are converted to sodium fluoride by the HF content of the offgases. Also, upon cooling, the sodium containing vapors may at first liquefy, then the liquid phase NaF may solidify as extremely fine particles. The condensation of the NaF causes the coating of equipment surfaces resulting in pluggages and the finely divided NaF product is difficult to separate in an efficient and economic manner from the gas stream and from the other dusts carried by the gas stream. In many instances, generation of NaF is not desired; often it is preferred that the offgas would contain, besides the gaseous combustion products of the spent lining, essentially only HF without other fluoridic products.
The prior art has already made recommendations for the extension of contact or reaction times in fluidized bed reactors. These recommendations have included the use of multizone reactors, wherein several physically separated stages are maintained. The gases emanating from the first stage bed of the reactor contact during their upward travel one or more additional fluid beds. These systems are particularly useful for the calcination of ores or other materials, such as alumina, but, unfortunately, when applied to the pyrohydrolysis of spent cell linings, they fail to provide the desired results due to the conditions existing in pyrohydrolysis units.
Conducting the pyrohydrolysis in a multistage or multizone reactors, such as referred to above, entails the usual mechanical and operational problems associated with the installation and operation of two or more independent fluid beds. In addition, since there is no generation of heat in the subsequent beds, it will be difficult, if not impossible, to maintain these beds at the same temperature as the initial pyrohydrolysis bed. A drop in temperature of the offgases from the pyrohydrolysis bed will condense sodium fluoride and results in pluggage of the distributor plates of the subsequent fluid beds. A drop in temperature will also adversely affect the capture of sodium oxide by the alumina and result in the recombination of sodium oxide and HF to form additional sodium fluoride. The same applies to fluidized bed systems which employ two side-by-side fluidized beds. In none of these existing systems can the reversion of the generated HF to NaF be avoided.
It has now been found that the conversion of the NaF constituent to HF and the conversion of the Na-containing vapors to Na.sub.2 O.xAl.sub.2 O.sub.3 can be readily and efficiently accomplished by introducing in the fluidized bed reactor a relatively finely divided source of Al.sub.2 O.sub.3, which will then react with the Na-containing vapors generated by the pyrohydrolysis of spent cell linings in the reactor. Contacting of the Na-containing vapors with the Al.sub.2 O.sub.3 source is preferably accomplished in the immediate vicinity of the fluidized bed. This assures the extension of the reaction time between the vapors, including the steam and the desired conversion of the NaF constituent of the vapors to HF.